Purpose of the anti-alias filter

The Delta44 has (of course) its own built-in anti alias filter.
Since the board is desined for music aliasing, near 48 kHz is
not an issue - the filter is not designed for an extreme cut-off.
It is designed for amplitude flatness and phase response at frequencies
well below 48kHz.

One could perfectly well use the signal directly from the mixer
to feed the Delta44, but then it will not be possible to tolerate
signals above the level that saturates Delta44 in the region from the
passband edge to as far outside (50kHz or so) as required for the
RF filters to attenuate the signals.
Without anti-alias filter the useful bandwidth is limited to about
43kHz because of aliasing.

The anti-alias filter incorporated in the 2.5MHz to audio converter
has some gain below 48kHz and attenuation above.
The mixer saturates 17dB above the Delta44 which means that the
filter extends the dynamic range by 17dB for frequencies more than
1kHz outside the visible passband.
(In the frequency range 48 to 49 kHz saturation of the Delta44
is the limiting factor)
The attenuation at 47kHz is about 6dB so the useful passband is
extended to 47kHz by this filter.

Schematic diagram and frequency response

The schematic diagram is shown in fig. 1 and the frequency response
in fig. 2.

Fig.1.
Schematic diagram of the anti-alias filter.

Fig.2.
Frequency response of the anti-alias filter from input to output.

Individual stages

The first stage is a notch filter followed by an integrator.
There is some feedback/feedforward to the second stage to improve
the attenuation between the two notch frequencies and to make
the corner at 48kHz sharper.

The frequency response at the output of the first stage is
shown in fig. 3.
Frequencies above 48kHz are attenuated by 6dB or more so there is
no risk for signals above 48kHz to saturate this stage which
is capable of the same voltage swing as the input stage.

In the schematic diagram the notch filter coils are marked
2mH and 20mH respectively. The details of these coils are as
follows:

The "2mH" coil is 100.5 turns of 0.20mm enameled wire on an
RM6 core of material N48 with an Al-value of 160.
The core is not critical, other materials and Al
values will also work fine. The coil has to have a parallel
resonance at 55kHz when parallel with 4.7nF.

The "20mH" coil has to have a series resonance with 470pF
at 49.2kHz.
It is wound with 340 turns of 0.10mm wire, also on an RM6
core with Al=160 made from N48 material.
Also for this core the Al-value and material is not critical.

Fig.3.
Frequency response at the output of the first stage.

The second stage is a a third order low-pass filter of Sallen/Key type.
The frequency response at the output of the second stage is shown
in fig. 4.

Fig.4.
Frequency response at the output of the second stage.

The third stage is similar to the second stage and it is added for
better suppression of spurs caused by extremely strong signals
in the range 20 to 100kHz from the visible spectrum edge.

The last stage is a differential amplifier which is needed to
reference the output signal to the analog ground of the Delta44.
The analog ground must not be current loaded so it can not be connected
to the ground of the 2.5 to audio converter.

Spur suppression for extremely strong signals outside the passband

A very strong signal from an old vacuum tube signal generator was
swept from 1 kHz outside the passband, 2.549kHz and upwards.
The signal level was +1.4dBm, 15dB above Delta44 saturation or
about 160dBc/Hz.

Fig.5.
Waterfall graph while a very strong signal (level=140dB on dB scale)
is swept from 2.046 to about 2.090MHz.
The white area at the bottom is due to Delta44 saturation below
2.048kHz.
The white line near the top is when the signal generator is switched off.
Fig.6.
Channel 1 Q. (For component numbers.)